Based on today's technology in direct drive generators, each coil is composed of more than one series turn while a chosen number of coils may also be connected in series. The two aforementioned selectable numbers, i.e. the number of series in turn and the number of coils connected in series, may be chosen to obtain the terminal voltage of the generator which itself may have already been chosen based on power electronics/grid requirements. In the described conventional type of winding, the series turns in each slot need to be electrically insulated from each other due to the voltage difference between the turns in series. Furthermore, the whole conductors in the slot need to be insulated from the neighbouring lamination via so called slot insulation due to the voltage difference between the turns and the lamination which is grounded through the coupling with shaft and wind tower.
The aforementioned type of insulation used in the slot of the machines with conventional windings, results in different disadvantages: Poor heat transfer coefficient of the insulation makes it very difficult for the main source of heat in the generator, i.e. the windings to get cooled down through the neighbouring laminations. Using the required insulation in the slots decreases the slot space for the active material, which is usually copper, and thereby the so called fill factor. This in turn reduces the out put torque for the same current density or decreases the efficiency for the same torque. A typical value of fill factor for conventional type of winding is in the range of 70-80%.